1. Field of the Invention
The present invention relates to a laser exposure apparatus and a laser exposure method, which use a laser light source formed by a multi-transverse mode semiconductor laser. The present invention also relates to a laser annealing apparatus and a laser annealing method, which use a laser light source formed by a multi-transverse mode semiconductor laser.
2. Description of the Related Art
In the field of thin film transistors (TFT), or the like, laser annealing is conducted, in which an amorphous semiconductor film, such as an amorphous silicon film, is exposed to laser light to be annealed, thereby obtaining the polycrystalline semiconductor film.
As conventional laser light sources for use in laser annealing, excimer lasers have been widely used in view of high power and good light absorption by amorphous semiconductor films. In recent years, use of a solid-state laser or a semiconductor laser as the laser light source has been considered in view of easier maintenance and handling. Among solid-state lasers and semiconductor lasers, high power, multi-transverse mode semiconductor lasers are preferred.
In order to form a polycrystalline semiconductor film having a uniform and high mobility by the laser annealing, it is important to have a uniform intensity distribution of the laser light applied to the amorphous semiconductor film.
In conventional laser annealing apparatuses using excimer lasers, an optical homogenizing means such as a beam homogenizer is used to homogenize the intensity distribution of the laser light applied to the substance to be exposed. The excimer laser simultaneously emits multiple high-order transverse modes and has a relatively low spatial coherence. Therefore, the intensity distribution of the laser light applied to the substance to be exposed can be satisfactorily homogenized only by providing the optical homogenizing means in the optical path.
In contrast, the number of high-order transverse modes simultaneously emitted by a multi-transverse mode semiconductor laser is smaller than those emitted by the excimer laser, and the multi-transverse mode semiconductor laser has a higher spatial coherence than the excimer laser. Therefore, the intensity distribution of the laser light applied to the substance to be exposed cannot be homogenized simply by providing the optical homogenizing means in the optical path. This is because that high-order transverse mode light of a certain order emitted by the multi-transverse mode semiconductor laser has two wavefront components that propagate in substantially symmetrical directions with respect to the optical axis, and the two wavefront components interfere with each other.
Japanese Unexamined Patent Publication No. 2003-347236 discloses a laser exposure apparatus, wherein laser light emitted from a single laser light source is divided into multiple beams, and a difference in optical path length between the divided multiple beams is set to be not less than the coherence length of the laser light, so that the multiple beams do not interfere with each other. Then, the multiple beams are combined to be applied to the substance to be exposed. It should be noted that the invention of Japanese Unexamined Patent Publication No. 2003-347236 makes the divided multiple laser beams incoherent with each other, and it does not reduce coherency of the multi-transverse mode laser light.